AsEjs does pretty much the same thing that EJS templates templates do, with the following delicate differences:

Streaming: Rather than rendering the template completely into a buffer first, and then outputting that buffer to the client,
AsEjs templates output chunks of their content directly and immediately into the output stream as soon as that makes sense.

Asynchronous: AsEjs support calling asynchronous methods from within the template,
i.e. methods that don't do anything at the time, but return their result at a later time using a callback function.
AsEjs templates understand this paradigm and pause the output stream at this point until the callback method gets called.
The rendering continues normally after that.

Both features, combined with Asynchronous Futures,
allow a web server that uses AsEjs templates to output the static headers of their response
immediately to the client browser, while waiting until data from the database is loaded. The remaining parts of the response are rendered
in chunks as soon as the required data becomes available from the backend.
This gives the browser a chance to prefetch static assets of the web page (CSS, JavaScript) in parallel while the
server generates the remaining response.